Exhaust gas purification apparatus for an internal combustion engine

Abstract

In order to supply a reducing agent to an NOx catalyst in an appropriate manner when the temperature of the NOx catalyst is high, in cases where the temperature of the NOx catalyst is equal to or higher than a predetermined temperature at which ammonia is oxidized, and in cases where an NOx reduction rate in the NOx catalyst is smaller than a predetermined lower limit reduction rate, when the addition of the additive agent is carried out at least twice by changing an equivalent ratio with respect to an amount of NOx flowing into the NOx catalyst, the equivalent ratio in equivalent ratio control is made larger in the case where an amount of change of an amount of reduction of NOx accompanying a change of the equivalent ratio is larger than an amount of change of an amount of oxidation of ammonia accompanying the change of the equivalent ratio, than in the case where it is smaller than that.

The invention claimed is: 
     
       1. An exhaust gas purification apparatus for an internal combustion engine comprising:
 an NOx selective catalytic reduction catalyst that is arranged in an exhaust passage of the internal combustion engine and selectively reduces NOx in an exhaust gas by using ammonia as a reducing agent; 
 an addition valve that is arranged in said exhaust passage at the upstream side of said NOx selective catalytic reduction catalyst and configured to add an additive agent, which is ammonia or a precursor of ammonia, into the exhaust gas; 
 a temperature sensor configured to acquire a temperature of said NOx selective catalytic reduction catalyst; and 
 a controller configured to carry out equivalent ratio control in which an amount of the additive agent corresponding to an amount of NOx flowing into said NOx selective catalytic reduction catalyst and an equivalent ratio is added from said addition valve; 
 wherein in cases where the temperature acquired by said temperature sensor is equal to or higher than a predetermined temperature at which ammonia is oxidized, and in cases where an NOx reduction rate in said NOx selective catalytic reduction catalyst is smaller than a predetermined lower limit reduction rate, when the addition of the additive agent from said addition valve is carried out at least twice by changing the equivalent ratio with respect to the amount of NOx flowing into said NOx selective catalytic reduction catalyst, said controller configured to make the equivalent ratio in said equivalent ratio control larger in the case where an amount of change of an amount of reduction of NOx accompanying a change of the equivalent ratio is larger than an amount of change of the amount of oxidation of ammonia accompanying the change of the equivalent ratio, than in the case where the amount of change of the amount of reduction of NOx accompanying the change of the equivalent ratio is smaller than the amount of change of the amount of oxidation of ammonia accompanying the change of the equivalent ratio. 
 
     
     
       2. The exhaust gas purification apparatus for the internal combustion engine as set forth in  claim 1 , wherein
 in cases where said amount of change of the amount of reduction of NOx is larger than said amount of change of the amount of oxidation of ammonia, said controller is configured to make the equivalent ratio in said equivalent ratio control larger than 1. 
 
     
     
       3. The exhaust gas purification apparatus for the internal combustion engine as set forth in  claim 2 , wherein
 in cases where said amount of change of the amount of reduction of NOx is larger than said amount of change of the amount of oxidation of ammonia, said controller is further configured to adjust the equivalent ratio in said equivalent ratio control in such a manner that the concentration of ammonia in the exhaust gas flowing out from said NOx selective catalytic reduction catalyst becomes equal to or less than a predetermined upper limit concentration. 
 
     
     
       4. The exhaust gas purification apparatus for the internal combustion engine as set forth in  claim 2 , wherein
 said controller is further configured to adjust the equivalent ratio in said equivalent ratio control in such a manner that the NOx reduction rate in said NOx selective catalytic reduction catalyst becomes equal to or more than said predetermined lower limit reduction rate. 
 
     
     
       5. The exhaust gas purification apparatus for the internal combustion engine as set forth in  claim 1 , wherein
 in cases where said amount of change of the amount of reduction of NOx is equal to or less than said amount of change of the amount of oxidation of ammonia, said controller is further configured to make the equivalent ratio in said equivalent ratio control equal to or less than 1.